There are numerous proposed schemes by which various natural ores, concentrates and slags may be processed for the recovery of metal values. One commercial scheme for the extraction and separation of tantalum and niobium (also known as columbium) values from ores, or from the widely available tin slags, is described in detail in U.S. Pat. Nos. 2,767,047, 2,953,453, 3,117,833, 3,300,297, 3,658,511, 3,712,939 and 4,164,417. In this process, feed solids are digested with hot concentrated hydrofluoric acid to solubilize most of the tantalum and niobium values as chlorides. A liquor containing the dissolved metal values is separated by filtration and treated in a multistage liquid—liquid extraction cascade wherein the metal values are extracted with methyl isobutyl ketone. The resulting filter cake from this process still contains small amounts (1-5%) of tantalum and niobium and also small amounts of natural radioactive elements such as uranium and thorium. Also retained in these solids is about 35 to 40% moisture and a small amount of residual hydrofluoric acid. Most of the metallic elements in the solids have been converted to fluorides or oxyfluorides during the initial digestion.
Generally the resulting filter cakes are stored for later processing to enable the further recovery of previously unrecovered metal values. Recovery of the remaining tantalum and niobium helps to offset the cost of treatment of these solids and also greatly reduces the amount of residual fluoride which might be undesirable in subsequent uranium recovery. Because of the variety of original ore sources and possible changes in the chemistry of the solids over time, any processing scheme must take into account the variable nature of these solids.
U.S. Pat. No. 5,023,059, the disclosure of which is hereby incorporated herein by reference, relates to a process for recovering metal values and hydrofluoric acid from the filter cake produced by ore processing.